1. Field of the Invention
The present invention relates to a metal laminate for a circuit board, having a film (which film is hereinafter referred to as a thermotropic liquid crystal polymer film) made of a thermotropic polymer capable of forming an optically anisotropic melt phase (which thermotropic polymer is hereinafter referred to as a thermotropic liquid crystal polymer), and a method for producing the same. More particularly, the metal laminate for a circuit board according to the present invention has not only an excellent low moisture absorbability, heat resistance, chemical resistance, and electric properties deriving from the thermotropic liquid crystal polymer film but also an excellent dimensional stability, so that it is useful as a material for a circuit board such as a flexible wiring board or a circuit board for mounting a semiconductor.
2. Description of the Prior Art
In recent years, the demands for scale reduction and weight reduction in portable electronic devices for mobile communication and others are becoming high, and an expectation for high density mounting is becoming increasingly strong. In accordance therewith, multi-layered wiring boards, reduced wiring pitches, fine via holes, and small-size multiple-pin IC packages are being developed, and also the scale reduction and surface mounting of passive elements such as capacitors and resistors are taking place along with this. Especially, the technique of forming these passive components directly on a surface or in an inside of a printed wiring board or the like can not only achieve high density mounting but also contributes to an improvement in reliability. In accordance therewith, the level of demands for the dimensional stability of the wiring boards. i.e. small variation in the dimension after the formation of a conductor circuit, are becoming high, and further there is a demand for eliminating its anisotropy.
In the meantime, thermotropic liquid crystal polymer films having an excellent low moisture absorbability, heat resistance, chemical resistance, and electric properties are rapidly commercialized as an electrically insulating material that improves the reliability of printed wiring boards and others.
Conventionally, in producing a metal laminate for use in a circuit board such as a printed wiring board using a thermotropic liquid crystal polymer film, a thermotropic liquid crystal polymer film cut to a predetermined size and a metal foil are placed in superposition between two hot platens with the use of a vacuum hot press device, and thermally press-bonded in a vacuum state (batch type vacuum hot press lamination). At this time, if the segment orientation ratio SOR of the thermotropic liquid crystal polymer film before press-bonding is approximately 1, then a metal laminate having a good dimensional stability is obtained. However, since the vacuum hot press lamination is a sheet-type production method, the period of time for superposing the materials, the period of time for one pressing operation, the period of time for taking out the material after pressing, and the like will be long, thereby slowing the production speed per one sheet of the metal laminate and leading to increased costs. Moreover, if the equipments are improved so that a large number of sheets can be produced at the same time for increasing the production speed, then the equipments will be large in scale, disadvantageously leading to high equipment costs. Accordingly, there is a demand for solving this problem and developing a continuous production method capable of providing metal laminates at a low cost.
Thus, there is proposed (a) a method of superposing a long thermotropic liquid crystal polymer film on a metal foil and allowing them to pass between hot rolls for press-bonding in such a manner that the press-bonding temperature is within the range from a temperature lower by 80xc2x0 C. than the melting point of the thermotropic liquid crystal polymer film to a temperature lower by 5xc2x0 C. than the melting point for performing continuous production of the metal laminates (Japanese Laid-open Patent Publication No. 05-42603/1993). Further, there is proposed a method of thermally treating a thermotropic liquid crystal polymer film at a predetermined temperature (Japanese Laid-open Patent Publication No. 08-90570/1996).
However, according to the methods of (a) and (b), it is difficult to continuously and stably obtain a metal laminate having a good isotropy and good dimensional stability. In other words, in the case where a thermotropic liquid crystal polymer film is press-bonded to a metal foil between hot rolls, the temperature conditions are described as shown above, but the tension imposed on the film in press-bonding is not considered at all. If this tension is applied, the movement of the molecules in the thermotropic liquid crystal polymer film is likely to occur. As a result, in accordance with heating, the change in the molecule orientation is likely to take place on the surface of the film in a metal laminate using the film. For these reasons, it has been difficult to obtain a metal laminate having a good isotropy and good dimensional stability.
Further, in the method of (a), although the conditions for improving the adhesion strength to the metal sheet and the improvement in the mechanical strength are described, the improvement in the dimensional stability is not described. In the method of (b), although the heated dimensional changing ratio of the thermotropic liquid crystal polymer film is described, the properties of the metal laminate using the thermotropic liquid crystal polymer film are not described. Therefore, in the conventional methods, continuous production of metal laminates for circuit boards having a good isotropy and a good dimensional stability has not been realized.
Accordingly, the object of the present invention is to provide a metal laminate for a circuit board and a method for producing the same by which the metal laminate for a circuit board having an excellent isotropy and an excellent dimensional stability can be produced continuously at a high productivity by using hot rolls and a heating equipment.
In order to achieve the aforesaid object, the inventors of the present invention have made studies and found a method that can continuously and stably produce metal laminates for a circuit board having an excellent isotropy and an excellent dimensional stability. The methods comprises the steps of: using a thermotropic liquid crystal polymer film whose segment orientation ratio SOR is within a specific range and; after the thermotropic liquid crystal polymer film and a metal sheet such as represented by a metal foil and a metal plate are press-bonded together between hot rolls under a specific tension condition depending on the segment orientation ratio SOR, heating the obtained laminate under a specific temperature condition. According to this method, the isotropy of a thermotropic liquid crystal polymer film in a laminated state can be obtained thereby obtaining a metal laminate for a circuit board being excellent in isotropy and dimensional stability.
The production method according to the first invention comprises a first step of using a thermotropic liquid crystal polymer film having a segment orientation ratio SOR within a range not smaller than 1.03 and smaller than 1.15 along a longitudinal direction of the film, and press-bonding a metal sheet on at least one surface of the thermotropic liquid crystal polymer film between hot rolls while said thermotropic liquid crystal polymer film is in a tense state; and a second step of heating the laminate obtained in the first step to a temperature not lower than a melting point of the thermotropic liquid crystal polymer film.
Here, the term xe2x80x9csegment orientation ratio (SOR)xe2x80x9d used hereinbefore and hereinafter is an index descriptive of the degree of orientation of molecules forming a segment and represents, unlike the standard MOR (molecular orientation ratio), a value in which the thickness of an object is taken into consideration. Further, the tense state refers to the state in which a tension (for example, 1.2 to 2.8 kg/mm2) is applied onto a film along the longitudinal direction of the film (tensile direction).
According to the knowledge of the present inventors, the thermotropic liquid crystal polymer film, when heated in a freely expandable and contractible state, contracts in the oriented direction of the molecules, and expands in the non-oriented direction, and also has a property such that the orientation direction changes easily in accordance with the force acting on the molecules.
According to the first invention, if the thermotropic liquid crystal polymer film has a segment orientation ratio SOR not smaller than 1.03 and smaller than 1.15 to be oriented in a longitudinal direction of the film, the film tends to contract in the longitudinal direction due to the above-mentioned property, and this contracting force brings the film into a tense state. However, by applying a tension to the film, the contracting force of the film is canceled, so that the orientation of the film in the longitudinal direction does not change. On the other hand, since an expanding force is applied to the film in a direction perpendicular to the longitudinal direction of the film, the orientation of the film changes in the direction perpendicular to the longitudinal direction due to the property such that the orientation direction is easily changed by the force acting on the molecules in the thermotropic liquid crystal polymer film. As a result of this, the film in a laminated state will be isotropic due to canceled anisotropy. By successively heating the obtained laminate to a temperature not lower than the melting point of the thermotropic liquid crystal polymer film, a metal laminate having an isotropic property and a desired dimensional changing ratio and being excellent in dimensional stability can be obtained continuously in a stable manner.
Further, the production method according to the second invention comprises a first step of using a thermotropic liquid crystal polymer film having a segment orientation ratio SOR within a range not smaller than 0.90 and smaller than 1.03 along a longitudinal direction of the film, and press-bonding a metal sheet on at least one surface of the thermotropic liquid crystal polymer film between hot rolls while said thermotropic liquid crystal polymer film is in a non-tense state; and a second step of heating the laminate obtained in the first step to a temperature not lower than a melting point of the thermotropic liquid crystal polymer film.
Here, the non-tense state refers to a state in which a tension higher than the gravitational force applied to the film by self weight is not applied to the film.
According to the second invention, if the thermotropic liquid crystal polymer film has a segment orientation ratio SOR not smaller than 0.90 and smaller than 1.03 to be generally oriented in a direction perpendicular to a longitudinal direction of the film, the film tends to contract in the direction perpendicular to the longitudinal direction, along which the molecules are oriented, and tends to expand in the longitudinal direction of the film, along which the molecules are not oriented, due to the above-mentioned property. In this case, by bringing the film into a non-tense state contrary to the first invention, i.e. by not applying a tension higher than the self weight to the film, the orientation direction changes to the longitudinal direction of the film. As a result of this, the film in a laminated state maintains the isotropy or becomes isotropic by cancellation of the anisotropy. By successively heating the obtained laminate to a temperature not lower than the melting point of the thermotropic liquid crystal polymer film, a metal laminate being excellent in isotropy and dimensional stability can be obtained in a stable manner.
Thus, the first and second inventions make it possible to continuously produce a metal laminate for a circuit board being excellent in isotropy and dimensional stability, thereby increasing the productivity and reducing the production costs.
The raw materials of the thermotropic liquid crystal polymer film to be used in the present invention are not specifically limited. Nonetheless, specific examples thereof include, for example, well known thermotropic liquid crystal polyesters and thermotropic liquid crystal polyester amides prepared from compounds, such as classified under (1) to (4) below, and their derivatives. It is, however, pointed out that to prepare a polymer capable of forming an optically anisotropic melt phase, various raw material compounds have their proper combination and amount carefully chosen.
(1) Aromatic or aliphatic dihydroxy compounds, representative examples of which are shown in Table 1 below.
(2) Aromatic or aliphatic dicarboxylic acids, representative examples of which are shown in Table 2 below.
(3) Aromatic hydroxycarboxylic acids, representative examples of which are shown in Table 3 below.
(4) Aromatic diamines, aromatic hydroxyamines, and aromatic aminocarboxylic acids, representative examples of which are shown in Table 4 below.
Representative examples of the thermotropic liquid crystal polymers prepared from any of those raw material compounds include copolymers (a) to (e) having such structural units as shown in Table 5 below.
Further, in order to impart a desired heat resistance and processability to the film, the thermotropic liquid crystal polymer to be used in the present invention has a melting point preferably within the range from about 200 to about 400xc2x0 C., more preferably within the range from about 250 to about 350xc2x0 C. However, in view of the film production, the thermotropic liquid crystal polymer preferably has a comparatively low melting point. Therefore, if a higher heat resistance or a melting point is needed, the obtained film can be further heated to raise the heat resistance and the melting point of the thermotropic liquid crystal polymer film to desired ones. An example of the heating condition will be hereafter explained. Even if the melting point of the obtained thermotropic liquid crystal polymer film is 283xc2x0 C., the film can be further heated at 260xc2x0 C. for 5 hours to achieve a melting point of 320xc2x0 C.
The thermotropic liquid crystal polymer film to be used in the present invention is obtained by extrusion-molding the aforementioned polymer. At that time, any extrusion molding method can be used. However, the T-die film-forming and stretching method, the laminate stretching method, the inflation method, and others well known in the art are industrially advantageous. Particularly with the inflation method, stresses can be applied not only in a direction of the mechanical axis (longitudinal direction) of the film (which direction is hereinafter referred to as the MD direction), but also in a direction (hereinafter referred to as the TD direction) perpendicular to the MD direction and, therefore, the inflation method is effective to eventually manufacture the thermotropic liquid crystal polymer film having balanced physical and thermal properties in both of the MD and TD directions.
The aforesaid thermotropic liquid crystal polymer film must have a segment orientation ratio (SOR) within the range not smaller than 0.90 and smaller than 1.15 in the longitudinal direction of the film. The thermotropic liquid crystal polymer film within this range has balanced physical and thermal properties in the aforesaid MD and TD directions, so that it not only has a high practicability but also advantageously provides good isotropy and dimensional stability of the metal laminate for a circuit board as described above.
The aforesaid segment orientation ratio SOR can be calculated in the following manner.
Using a commercially available microwave molecular orientation degree measuring apparatus, a thermotropic liquid crystal polymer film is inserted into a microwave resonance waveguide so that the film surface thereof is perpendicular to the propagation direction of the microwave, and the intensity (the microwave penetration strength) of an electric field of the microwave transmitted through the thermotropic liquid crystal polymer film is measured.
Based on the resultant measurement, an m value (hereinafter referred to as an xe2x80x9cindex of refractionxe2x80x9d) can be calculated from the following equation:
m=(Z0/xcex94z)xc3x97(1xe2x88x92xcexdmax/xcexd0)
wherein Zo represents a device constant, xcex94 z represents an average thickness of an object subjected to the measurement, xcexdmax represents the frequency at which the maximum microwave penetration strength can be obtained when the frequency of the microwave is changed, and xcexdo represents the frequency at which the maximum microwave penetration strength can be obtained when the average thickness is zero, that is, when no object is present.
The segment orientation ratio (SOR) can be calculated from the following equation:
SOR=mo/m90
wherein m0 represents a value of the m value which is exhibited when the angle of rotation of the object relative to the direction of oscillation of the microwave is 0xc2x0, that is, when the direction of oscillation of the microwave is aligned with the direction in which molecules of the object are most oriented (generally the longitudinal direction of the extrusion-molded film) and in which the minimum microwave penetration strength is exhibited, and m90 represents a value of the m value which is exhibited when the angle of rotation of the object is 90xc2x0.
If the segment orientation ratio SOR is not greater than 0.50 or not smaller than 1.50, the deviation of the orientation of the liquid crystal polymer molecules is considerable, so that the film will be hard and is likely to be torn in the TD direction or in the MD direction. For use in a circuit board requiring a morphological stability such as absence of warping in heating, it is necessary that the segment orientation ratio SOR is within the range not smaller than 0.90 and smaller than 1.15. Particularly, if the warping at the time of heating must be almost completely eliminated, the segment orientation ratio SOR is preferably not smaller than 0.95 and not greater than 1.08.
The thermotropic liquid crystal polymer film to be used in the present invention may have an arbitrary thickness, and includes plate-like or sheet-like films having a thickness not greater than 2 mm. However, if a metal laminate using a thermotropic liquid crystal polymer film as an electrically insulating material is to be used as a printed wiring board, the thickness of the film is preferably within the range from 20 to 150 xcexcm, more preferably within the range from 20 to 50 xcexcm. If the thickness of the film is too small, the rigidity and the strength of the film will be small, so that the film is deformed in mounting electronic components on the obtained printed wiring board, thereby deteriorating the position precision of the wiring to cause a failure of the circuit. Also, as an electrically insulating material for a main circuit board, for example, of a personal computer or the like, it is possible to use a composite of the aforesaid thermotropic liquid crystal polymer film with another electrically insulating material such as glass cloth base material. Further, additives such as a lubricant and an antioxidant may be blended with the film.
Further, in the present invention, in the first step, a long metal sheet is superposed on at least one surface of a long thermotropic liquid crystal polymer film, and these are press-bonded between hot rolls for forming a laminate. The hot rolls may be, for example, a pair of a heat-resistant rubber roll and a hot metal roll in the case of a single-sided metal laminate. The heat-resistant rubber roll and the metal roll are preferably positioned in such a manner that the heat-resistant rubber roll is positioned at the film side, and the metal roll is positioned on the metal sheet side. In the case of a double-sided metal laminate, a pair of hot metal rolls are used. By transporting the thermotropic liquid crystal polymer film and the metal sheet in a superposed state in the longitudinal direction of the film, they are supplied between the rolls to thermally press-bond the film to the metal sheet for forming a laminate. In a subsequent second step, the metal laminate is heated to a temperature not lower than the melting point of the thermotropic liquid crystal polymer film to form a metal laminate for a circuit board.
The heat-resistant rubber rolls used in the case of the aforesaid single-side metal laminate preferably have a hardness of the roll surface of 80 degrees or more, more preferably within the range from 80 to 95 degrees, as tested by an A-type spring hardness tester based on JIS K6301. The rubber of 80 degrees or more is obtained by adding a vulcanization accelerator such as a vulcanizer or an alkaline substance into a synthetic such as a silicone rubber or a fluororubber, or natural rubber. At this time, if the hardness is smaller than 80 degrees, the pressure insufficiency is invited at the time of thermal press-bonding, thereby causing insufficient adhesion strength of the laminate. On the other hand, if the hardness exceeds 95 degrees, a local line pressure is applied between the hot metal roll and the heat-resistant rubber roll, raising a fear that the appearance of the laminate will be poor.
As described above, after the thermotropic liquid crystal polymer film and the metal sheet are thermally press-bonded, the obtained laminate is heated to a temperature not lower than the melting point of the thermotropic liquid crystal polymer film in the second step, thereby to form the metal laminate. Here, since the thermal expansion coefficient of the thermotropic liquid crystal polymer film changes at the time of thermal press-bonding with the metal sheet and at the time of heating the laminate, a process must be designed taking this change into consideration in advance. As an example thereof, in the thermal press-bonding with the metal sheet in the first step, the tension applied to the thermotropic liquid crystal polymer film is preferably adjusted to be within the range from 1.2 to 2.8 kg/mm2. Further, in the second step, if the laminate is to be heated successively to the first step, the tension applied to the laminate is preferably adjusted to be within the range from 2.5 kg to 5.5 kg for a width of 40 cm. Further, the means for heating the laminate in the second step is not specifically limited, and may be, for example, a hot air circulating dryer, a hot air heating furnace, a hot roll, a ceramic heater, or the like. For the purpose of preventing oxidation of the surface of the metal sheet, it is preferable to heat the laminate in an inert atmosphere having an oxygen concentration of 0.1% or less by using heated nitrogen gas.
The above-mentioned heating process is preferably performed at a temperature within the range from the melting point of the thermotropic liquid crystal polymer film to the temperature higher by 30xc2x0 C. than the melting point. If the heating temperature is lower than the melting point, the effect of improving the dimensional stability is inferior, and moreover, the adhesion strength with the metal sheet is low. On the other hand, if the heating temperature exceeds the temperature higher by 30xc2x0 C. than the melting point, it comes near to the decomposition temperature of the thermotropic liquid crystal polymer film, thereby disadvantageously causing deterioration of the appearance such as coloring.
The metal foil to be used in the present invention is not specifically limited, but is suitably a metal such as used for establishing electrical connection. For example, gold, silver, nickel, aluminum, and other metals may be mentioned in addition to copper. A copper foil may be any of those produced by calendering, electrolysis, and the like. However, those having a large surface roughness produced by electrolysis are preferable because their adhesion strength with the thermotropic liquid crystal polymer film is high. The metal foil may be subjected beforehand to a chemical treatment such as acid washing, which is generally performed on copper foils. The thickness of the metal foil is preferably within the range from 9 to 200 xcexcm, more preferably within the range from 9 to 40 xcexcm.
Further, in the present invention, a metal plate having a thickness within the range from 0.2 to 2 mm can be used instead of the metal foil. In particular, in the case where the laminate of the present invention is to be used as a heat-dissipating plate for electronic components, the thickness of the metal plate is preferably within the range from 0.2 to 1 mm in view of the bending processability. The metal plate having such a thickness is generally produced by calendering, and is flat with its surface roughness being not greater than 1 xcexcm, so that it is preferable to use the metal plate after chemically or physically making the surface roughness to be 2 to 4 xcexcm. By doing so, the adhesion strength between the metal plate and the thermotropic liquid crystal polymer film will be high. Further, the surface roughness is not particularly limited, but it is preferable to avoid a roughness of 50% or more of the thickness of the metal plate, because the strength of the metal plate will then be fragile. Further, it is preferable to avoid the surface roughness of 50% or more of the thickness of the thermotropic liquid crystal polymer film, because the strength of the thermotropic liquid crystal polymer film will then be fragile.